The present invention relates to a semiconductor device, and more particularly to a semiconductor device having an overvoltage protection circuit.
In recent years, it is demanded that vehicle-mounted products be designed with functional safety in mind. For example, ISO 26262 is a functional safety standard. The assurance of functional safety includes the following stages.
(1) Detecting an abnormality and bringing an operation to a stop. (2) Switching to a normal circuit upon detection of an abnormality and maintaining an operation.
Automakers and automotive components manufacturers are making efforts to achieve functional safety by first reaching stage (1) and then reaching stage (2) in the future.
Meanwhile, stringent cost requirements are imposed on a vehicle-mounted semiconductor integrated circuit (semiconductor IC). Thus, it is necessary that a small, low-voltage transistor be effectively used to form a small IC. Further, if the IC operates on a vehicle-mounted battery, it must work normally under the following conditions:
(1) A voltage of approximately 12.5 V is applied to the IC during a normal operation.
(2) The IC must be capable of operating at a low voltage when a crank pulse is input. For example, a battery voltage suddenly decreases to 3.9 V when recovery is achieved from idling.
(3) A surge voltage is applied to the IC in the event, for instance, of a load dump. For example, a surge voltage as high as 40 V is applied for a period as short as 0.4 s.
In order to perform normal operations under the above conditions, that is, in order to operate upon the application of a high voltage from a battery and use a low-voltage circuit formed of low-voltage elements, the IC incorporates a step-down circuit (e.g., a dropper circuit) based on a high-voltage transistor. In this instance, the low-voltage circuit in the IC operates on a constant voltage that is derived from a battery voltage by the step-down circuit. This ensures that the low-voltage circuit formed of low-voltage elements can use a high voltage applied from the battery.
A semiconductor device described, for instance, in Japanese Unexamined Patent Publication No. 2012-238693 uses a high-voltage element to control the supply of a power supply voltage derived from a battery voltage.
FIG. 1 is a diagram illustrating the configuration of the semiconductor device described in Japanese Unexamined Patent Publication No. 2012-238693. Referring to FIG. 1, the semiconductor device includes a to-be-protected circuit (low voltage circuit 800) and a protection circuit 700, which is disposed between the low-voltage circuit 800 and a power supply that supplies a power supply voltage VIN. The protection circuit 700 includes a P-channel MOS transistor 701, zener diodes 702, 704, and a resistor 703. The source and drain of the P-channel MOS transistor 701 are coupled between the power supply and the low-voltage circuit 800. The gate of the P-channel MOS transistor 701 is coupled to the power supply through the zener diode 702 and coupled to a reference power supply (e.g., GND) through the resistor 703. The anode of the zener diode 702 is coupled to the gate of the P-channel MOS transistor 701 and coupled to the reference power supply through the resistor 703. The cathode of the zener diode 702 is coupled between the power supply and the source of the P-channel MOS transistor. The cathode of the zener diode 704 is coupled between the low-voltage circuit 800 and the drain of the P-channel MOS transistor. The anode of the zener diode 704 is coupled to the reference power supply.
Under normal conditions, that is, when the power supply voltage VIN is not higher than a predetermined voltage, no current flows to the zener diode 702. Therefore, the gate-source voltage of the P-channel MOS transistor is equal to a reference power supply voltage (e.g., ground voltage) −VIN. As the P-channel MOS transistor is on, a power supply voltage VD supplied to the low-voltage circuit 800 is at substantially the same potential as the power supply voltage VIN.
Meanwhile, if the power supply voltage VIN rises above the predetermined voltage, the zener diode 702 breaks down. In this instance, the gate-source voltage of the P-channel MOS transistor 701 is clipped at the breakdown voltage of the zener diode 702. This limits the amount of current to be supplied from the P-channel MOS transistor 701 to the low-voltage circuit 800.
When the power supply voltage VIN rises, the power supply voltage VD also rises, thereby causing the zener diode 704 to break down. In this instance, the amount of current supplied from the P-channel MOS transistor 701 is set to be not larger than the amount of allowable current that can flow to the zener diode 704. This ensures that the power supply voltage VD is clipped at the breakdown voltage of the zener diode 704. In other words, even if the power supply voltage VIN unduly rises, the zener diode 704 clips the power supply voltage VD at a predetermined voltage, thereby protecting the low-voltage circuit 800.
Meanwhile, a semiconductor device described in Japanese Unexamined Patent Publication No. 2009-246347 switches the power supply voltage to be supplied to an internal circuit (functional circuit). As shown in FIGS. 1 and 2 of Japanese Unexamined Patent Publication No. 2009-246347, a protection circuit having a switching element and a resistive element is parallel-coupled between the functional circuit and a first potential supply terminal. If the potential difference applied between the first potential supply terminal and a second potential supply terminal is equal to a predetermined value, the switching element supplies a power supply voltage to the functional circuit. If, on the other hand, an overvoltage is applied, the resistive element supplies the power supply voltage to the functional circuit.